As is well known, the space and time coherency of radiation from lasers is much higher than those of "normal", thermal radiation sources (such as ordinary light bulbs). These characteristics of a laser beam are much valuable and have meant that optical technics during later years have found new applications important for the society, for instance through the CD-technique. Within the visible region of the electromagnetic spectra, in particular gas lasers, for instance the well-known HeNe and argon-ion lasers, exhibit very good space coherency, and the beam width from these lasers has rotation symmetrically Gaussian distribution, the wave fronts are uniform (plane or spherical) and free from astigmatism. Such a well-defined beam characteristics are for instance very valuable when gas lasers are used together with diffraction optical elements (DOE), for example kinoforms. For ideal operation, the beam which incides towards the kinoform must be well-defined and known.
Another laser type is a semiconductor laser (or a diode laser), which is very interesting due to its small size, its ability to modulate, its low power consumption and its long life. It is used in several applications where these characteristics are required. However, there are many applications where one wants to use diode lasers where today gas lasers are used. The reason that one does not changes to a diode laser is the poor beam quality, ellipticity at astigmatism and indefinite intensity distribution across the beam of the diode lasers. In many cases, the diode laser is ideal for use in connection with diffraction optical components. This specially applies to its smallness, which makes it possible to combine DOE and diode laser in small and compact units. However, the beam quality of the diode lasers is not ideal for being used together with DOE. The present invention ensures an ideal function of diode laser, first by improving the beam quality and then the beam may illuminate the diffraction optical component.
The conformity between the beam which actually leaves the kinoform and the beam which at design time of the kinoform is assumed to leave it, depends on how well the beam onto the kinoform conforms with the one which is assumed to incide the kinoform through its design. A well-defined beam, which incides the kinoform is consequently a requirement for a well-defined outgoing beam and thereby a basic important requirement to obtain high beam quality out from the kinoform. One example of a diffraction optical element which requires illumination with very high beam quality is a kinoform formed for beam shaping purposes. A kinoform consists of a translucent plate, preferably of plastic or glass. The light phase that passes the plate varies over its cross-section in a way that it is precalculated in a computer. The phase agitation is carried out by means of corresponding variations in the refractive index of the plate or by applying one side of the plate with a shallow, about 1 .mu.m thick surface relief. The kinoform uses the light up to 90%, which is important in a case where the light effect is moderate, for example some mW. In an ordinary production method a relief original is produced by means of an electronic beam or laser lithography, after which a first "master" of metal is manufactured. The latter is used to "stamp" the relief onto the relief carrier.
There are many ways to improve the quality of a laser beam, for instance through different types of filtration, for example focusing through a small aperture (1-100 .mu.m) or by means of a singlemode fibre. Depending on the quality of the laser beam, different degrees of filtration can be needed. Gas lasers, as mentioned earlier, have a high beam quality, however not ideal. There is always a small amount of scattered light. There is a need for a certain filtration to ensure the quality, also from a gas laser, for example by means of a small aperture. Diode lasers have much worse beam quality, which requires higher filtration, for example by means of a singlemode fibre.
The advantage of using a singlemode fibre is that one probably knows the out coming beam, which is well-defined mathematically.
Through a filter arrangement the kinoform can be illuminated with a known mathematically simple descriptive beam. A well-defined beam with a filter has also the advantage of simplifying computations when designing, the kinoform and eliminates need to measure the incide beam before the design of the kinoform. With "a well-defined beam" is meant that the beam has substantially Gaussian rotation symmetrical intensity cross-section. where the wave front is plane or spherical.